I agree that the performance of ExtJS 4 is not yet acceptable. If I gave you the impression that I think otherwise then I apologize. Sencha have also made it clear on a number of occasions that they see performance as the top priority for the next few releases.
Monkey-patching to solve performance issues is not the ideal solution but it's currently all we have. If you can afford to wait for Sencha to solve these issues then obviously there's no point trying to do it yourself. However, not everyone has the luxury of being able to wait and for those developers who desperately need to improve performance right now so that they can make a release it may be worth a shot at optimizing. There's no guarantees it will work, it's just something to consider. In many cases, like that of stahlman, it will prove fruitless and frustrating.